Integrated circuits (“IC's”) have become such an integral part of so many daily activities that the needs for IC's continue to demand higher performance, lower cost, increased miniaturization of components, and greater packaging density of IC's. As new generations of IC products are released, their functionality increases while the size and number of components decreases. These demands make the IC's susceptible to damage during manufacturing and in use. Commonly, a package is used which both protects the circuit and provides electrical interconnections to external circuitry. The package often encases the IC and its connectivity to the electrical interconnections.
IC devices are constructed from a silicon or gallium arsenide wafer through a process that comprises a number of deposition, masking, diffusion, etching, and implanting steps. Usually, many individual devices are constructed on the same wafer. When the devices are separated into individual rectangular units, each takes the form of an IC die. In order to interface a die with other circuitry, it is common to mount it on a leadframe or on a multi-chip module substrate that is surrounded by a number of lead fingers. Each die has bonding pads that are then individually connected in a wire-bonding operation to the leadframe's lead finger pads using extremely fine gold or aluminum wires. The assemblies are then packaged by individually encapsulating them in molded plastic or ceramic bodies.
IC packaging technology has shown an increase in semiconductor chip density (the number of chips mounted on a single circuit board or substrate) that parallels the reduction in the number of components that are needed for a circuit. This results in packaging designs that are more compact, in form factors (the physical size and shape of a device) that are more compact, and in a significant increase in overall IC density. However, IC density continues to be limited by the space (or “real estate”) available for mounting individual die on a substrate.
To condense further the packaging of individual devices, packages have been developed in which more than one device can be packaged at one time at each package site. Each package site is a structure that provides mechanical support for the individual IC devices. It also provides one or more layers of interconnect lines that enable the devices to be connected electrically to surrounding circuitry. Of importance to complicated packaging designs are considerations of input/output count, heat dissipation, matching of thermal expansion between a motherboard and its attached components, cost of manufacturing, ease of integration into an automated manufacturing facility, package reliability, and easy adaptability of the package to additional packaging interfaces such as a printed circuit board (“PCB”).
In some cases, multi-chip devices can be fabricated faster and more cheaply than a corresponding single IC chip, that incorporates all the same functions. Current multi-chip modules typically consist of a PCB substrate onto which a set of separate IC chip components is directly attached. Such multi-chip modules have been found to increase circuit density and miniaturization, improve signal propagation speed, reduce overall device size and weight, improve performance, and lower costs—all primary goals of the computer industry.
However, such multi-chip modules can be bulky. IC package density is determined by the area required to mount a die or module on a circuit board. One method for reducing the board size of multi-chip modules and thereby increase their effective density is to stack the die or chips vertically within the module or package.
Such designs are improvements over prior multi-chip package and system-in-a-package (“SiP”) designs that combined several semiconductor die and associated passive components (“passives”) side by side in a single, horizontal layer. Combining them into a single horizontal layer used board space inefficiently by consuming large substrate areas, and afforded less advantage in circuit miniaturization.
However, multi-chip modules, whether vertically or horizontally arranged, can also present problems because they usually must be assembled before the component chips and chip connections can be tested. That is, because the electrical bond pads on a die are so small, it is difficult to test die before assembly onto a substrate. Thus, when die are mounted and connected individually, the die and connections can be tested individually, and only known-good-die (“KGD”) that are free of defects are then assembled into larger circuits. A fabrication process that uses KGD is therefore more reliable and less prone to assembly defects introduced due to bad die. With conventional multi-chip modules, however, the die cannot be individually identified as KGD before final assembly, leading to KGD inefficiencies and assembly process problems including yield.
Despite the advantages of recent developments in semiconductor fabrication and packaging techniques, there is a continuing need for improved packaging methods, systems, and designs for increasing semiconductor die density in PCB assemblies.
Thus, a need still remains for an integrated circuit package on package system to provide improved reliability and manufacturing yield. In view of the increasing demand for improved density of integrated circuits and particularly portable electronic products, it is increasingly critical that answers be found to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.